Spherical alumina particles and production process thereof

ABSTRACT

A process for producing roundish alumina particles includes heating at 1,000 to 1,600° C. a composition containing at least one of electrofused alumina and sintered alumina having a mean particle size greater than 35 μm, and at least one species selected from the group consisting of a halogen compound, a boron compound and an alumina hydrate; and crushing the composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. § 111(a)claiming the benefit pursuant to 35 U.S.C. § 119(e) (1) of the filingdate of Provisional Application No. 60/295,817 filed Jun. 6, 2001pursuant to 35 U.S.C. §111 (b).

TECHNICAL FIELD

The present invention relates to roundish alumina particles and to anindustrial, economical process for producing alumina particles which areparticularly useful for materials such as sealing material forelectronic parts; fillers; finish lapping material; and aggregatesincorporated into refractory, glass, ceramic, or composite materialthereof and which cause little wear and exhibit excellent flowcharacteristics. The invention also relates to roundish aluminaparticles produced through the process and to a high-thermal-conductiverubber/plastic composition containing the alumina particles.

BACKGROUND ART

In recent years, demands for higher integration and higher density ofelectronic parts have elevated electric power consumption per chip.Thus, effective removal of the generated heat in order to suppresstemperature elevation of electronic elements is a critical issue. Inview of the foregoing, alumina, particularly corundum (α-alumina),exhibiting excellent thermal conductivity, has become a candidate fillerfor a heat-dissipation spacer; a substrate material on which insulatingsealing materials for semiconductors and parts of semiconductor devicesare mounted; etc., and modification of alumina has been effected in avariety of fields.

Among such corundum particles, JP-A HEI 5-294613 discloses non-hollow,spherical corundum particles having no fractures and a mean particlesize of 35 μm or less, the particles being produced by adding aluminumhydroxide and optional, known agents serving as crystallizationpromoters in combination to a pulverized product of alumina such aselectrofused alumina or sintered alumina, and firing the mixture.

However, the above publication provides no clear description aboutcorundum particles, in terms of the particle size, shape, etc., producedby use of electrofused or sintered alumina having a mean particle sizegreater than 35 μm.

There has been also known a thermal spraying method in which aluminaproduced through the Bayer method is atomized into high-temperatureplasma or oxygen-hydrogen flame so as to melt and quench, to therebyproduce roundish particles. Although the thermal spraying methodprovides coarse alumina particles having a size greater than 35 μm, unitheat energy requirement is large, resulting in high costs. In addition,the thus-produced alumina, though predominantly containing α-alumina,includes by-products such as δ-alumina. Such an alumina product is notpreferred, since the product fails to exhibit properties required foralumina; e.g., lower thermal conductivity.

Pulverized products of electrofused alumina or sintered alumina havebeen also known as corundum particles having a size greater than 35 μm.However, these corundum particles are of indefinite shape having sharpfractures and produce significant wear in a kneader, a mold, etc. duringincorporation thereof into rubber/plastic. Thus, these corundumparticles are not preferred from a standpoint of practical use.

Several methods for producing a rubber/plastic composition exhibitinghigh thermal conductivity have been known; e.g., a method in which ahigh-thermal-conductivity filler such as aluminum nitride, boronnitride, or silicon carbide is incorporated, and a method in which afiller is added in as large an amount as possible. However, the formermethod is economically disadvantageous, since incorporation of thehigh-thermal-conductivity filler involves considerably high costs ascompared with incorporation of α-alumina. When the latter incorporationmethod is employed, thermal conductivity of the resultant compound(composition) can be increased by virtue of incorporation of the fillerat high concentration. However, the compound exhibits poor plasticfluidity, causing difficulty in molding thereof. Thus, a limitation isimposed on the amount of filler.

There has been studied another method in which particle sizedistribution is widened by incorporating fillers having particle sizesfalling within a variety of ranges for enhancing plastic fluidity of thecompound and increasing the amount of filler. However, sinceself-aggregation force of a microparticle component increases withdecreasing particle size, fluidity is deteriorated upon incorporationthereof into rubber/plastic, and the component forms aggregatedparticles in the resultant rubber/plastic composition, possibly loweringthermal conductivity. Thus, a limitation is also imposed on the particlesize that allows use of microparticles. With regard to a coarse particlecomponent, coarse particles desirably have a nearly round shape forattaining high fluidity. However, as disclosed in JP-A HEI 5-294613,particles having a mean particle size generally greater than 35 μm andno fractures are difficult to produce.

In view of the foregoing, the present inventors have carried outextensive studies, and an object of the present invention is to providea filler made of alumina whose particles impart less grinding andpolishing performance and which can be incorporated in a large amount,through improvement of a process for producing roundish corundumparticles.

DISCLOSURE OF THE INVENTION

The present inventors provide a process for producing roundish aluminaparticles useful for solving the aforementioned problems; aluminaparticles produced through the process; and a high-thermal-conductivityrubber/plastic composition into which the alumina particles areincorporated.

Specifically, the present invention provides a process for producingroundish alumina particles, comprising heating at 1,000 to 1,600° C. acomposition containing at least one of electrofused alumina and sinteredalumina having a mean particle size greater than 35 μm, and at least onespecies selected from the group consisting of a halogen compound, aboron compound and an alumina hydrate; and crushing the composition.

In the process, the electrofused alumina and sintered alumina have amean particle size of at least 50 μm.

In the process, the electrofused alumina and sintered alumina have amean particle size greater than 35 μm and less than or equal to 120 μm.

In any one of the processes, at least one of the halogen compound andboron compound is added in an amount of 5 to 20 mass % based on anentire amount of alumina.

In any one of the processes, the halogen compound is at least onespecies selected from the group consisting of AlF₃, NaF, CaF₂, MgF₂ andNa₃AlF_(6.)

In, any one of the processes, the boron compound is at least one speciesselected from the group consisting of B₂O₃, H₃BO₃, mNa₂O.nB₂O₃, whereineach of m and n is an integer, and a borofluoride compound.

In any one of the processes, the alumina hydrate is at least one speciesselected from the group consisting of aluminum hydroxide, alumina gel,amorphous aluminum hydroxide and a partial hydrate of aluminum compound.

In any one of the processes, each of the electrofused alumina, sinteredalumina and alumina hydrate has an α-ray level of 0.01 c/cm²·hr or less.

The present invention further provides roundish alumina particlesproduced through any one of the production processes.

The present invention further provides a high-thermal-conductivityrubber composition containing the roundish alumina particles thusproduced.

The present invention further provides a high-thermal-conductivityplastic composition containing the roundish alumina particles thusproduced.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail.

The present invention provides a process for producing roundish aluminaparticles, comprising heating at 1,000 to 1,600° C. a composition atleast one of containing electrofused alumina and sintered alumina havinga mean particle size greater than 35 μm, and at least one speciesselected from the group consisting of a halogen compound, a boroncompound and an alumina hydrate; and crushing the composition.

The alumina coarse particles that are employed as a starting material inthe present invention may be a pulverized product of electrofusedalumina or that of sintered alumina. In either case, the pulverizedproduct is produced through any known method. The pulverized product ofelectrofused alumina or sintered alumina has a mean particle sizegreater than 35 μm, preferably 50 μm or more, more preferably greaterthan 35 μm and less than or equal to 120 μm, with 50 to 120 μm beingparticularly preferred. A mean particle size of 35 μm or less is notpreferred since the mean particle size of the roundish alumina particlesproduced from the starting material does not exceed 35 μm.

In order to enhance roundness of coarse particles, an alumina hydrateserving as a roundness enhancer may be added in advance to electrofusedalumina and/or sintered alumina in accordance with needs, followed byheating. Examples of the alumina hydrate used in the present inventioninclude aluminum hydroxides such as gibbsite, bayerite, boehmite anddiaspore; amorphous aluminum hydroxides such as alumina gel andpseudo-boehmite; and aluminum compound partial hydrates such as aluminumoxide (alumina) whose surface is partially hydrated. Of these, aluminumhydroxide, alumina gel, and alumina microparticles exhibiting highthermal reactivity are particularly preferred. From the viewpoint ofeconomy, aluminum hydroxide (gibbsite) produced through the Bayer methodis preferred, and that having a mean particle size of 10 μm or less ismost preferred.

The present inventors have observed a quite surprising phenomenon thatthe roundness enhancer acts on coarse alumina particles synergisticallywith below-mentioned other agents added in accordance with needs, andselectively acts on (or is absorbed by) irregular, sharp fractures, tothereby round coarse alumina particles.

No particular limitation is imposed on the amount of the roundnessenhancer, since the amount varies in accordance with the particle sizedistribution of a pulverized product of electrofused alumina or sinteredalumina or a similar factor. For example, when aluminum hydroxide isadded, the amount preferably falls within a range of 5 to 300 mass %based on electrofused alumina and/or sintered alumina, as reduced toalumina. More preferably, the amount falls within a range of 50 to 150mass %. When the aforementioned amount is less than 5 mass %, coheringforce of aggregates increases, whereas when the amount is in excess of300 mass %, excess aluminum hydroxide is released and possibly migratesas alumina microparticles into products.

With regard to other agents that are added prior to heat treatment inaccordance with needs, known compounds serving as crystal growthpromoters for alumina are used singly or in combination. Preferredcrystal growth promoters are halogen compounds preferably led by atleast one fluorine compound selected from the group consisting of AlF₃,NaF, CaF₂, Na₃AlF₆ and MgF₂, and/or at least one boron compound selectedfrom the group consisting of B₂O₃, H₃BO₃, mNa₂O.nB₂O₃, wherein each of mand n is an integer, and a borofluoride compound. Of these, acombination of the fluorine compound and the boron compound, and theborofluoride compound are particularly preferred.

When the particle size of electrofused alumina or sintered aluminaincreases, reduction of fractures decreases. When the amounts of theother agents added in addition to the aforementioned alumina hydrate areincreased, reduction of fractures is further promoted. Thus, even thoughcoarse electrofused alumina/sintered alumina having a mean particle sizegreater than 35 μm , which hitherto has not been employed, is used asstarting material, there can be produced coarse, roundish aluminaparticles which cause substantially no problematic wear of a kneaderduring incorporation thereof into rubber/plastic or a mold duringmolding.

Although the amount of agents to be added varies in accordance with theparticle size of employed sintered alumina/electrofused alumina, heatingtemperature, residence time in a furnace and the type of heatingfurnace, the effective concentration of added agents is preferably 3mass % or more based on the total alumina components, particularlypreferably 5 mass % or more. In addition, the concentration ispreferably 20 mass % or less. When the amount of added agents is lessthan 3 mass %, the effect of fracture reduction is insufficient, therebyincreasing wear of a kneader during incorporation thereof intorubber/plastic or a mold during. molding, whereas addition in an amountgreater than 20 mass % is not preferred from the viewpoint of economy.

The type of the heating furnace is not limited, and known means such asa single kiln, a tunnel kiln and a rotary kiln may be employed. Noparticular limitation is imposed on the heating temperature so long asthe temperature assures formation of α-alumina as a final product. Theheating temperature is typically 1,000° C. or higher, preferably 1,300°C. to 1,600° C. inclusive, more preferably 1,300° C. to 1,500° C.inclusive. When the temperature is elevated to 1,600° C. or higher,cohering force of aggregates increases even in the presence of aluminumhydroxide, thereby inhibiting crushing to primary particles. Therequired residence time in the heating furnace, which varies inaccordance with the heating temperature, is 30 minutes or longer,preferably approximately one hour to three hours.

The coarse alumina particles produced through the aforementioned methodtend to assume secondary aggregated particles. Thus, the particles arecrushed for a short period of time by means of a known pulverizationmeans such as a ball mill, a vibrating mill or a jet mill to therebyyield roundish corundum particles having a target particle sizedistribution.

In the aforementioned production method, roundish alumina particleshaving a low α-ray level can be produced from materials such aselectrofused alumina; sintered alumina and aluminum hydroxide, allcontaining trace amounts of radioactive elements such as uranium andthoria. Such roundish alumina particles having a low α-ray level (0.01c/cm²·hr) are particularly useful for preventing operation failure ofmemory devices (i.e., software error) caused by α-rays when theparticles are used as a filler of a resin sealing material forhigh-integration ICs, LSIs and VLSIs.

The roundish alumina produced according to the present invention assumesthe form of coarse corundum particles that, although some fracturesremain, provide no problematic wear of a kneader during incorporationthereof into rubber/plastic or a mold during molding and impartfavorable fluidization characteristics when used in combination with amicroparticle component.

The roundish alumina particles produced according to the process of thepresent invention are preferably incorporated into rubber or plastic tothereby provide a high-thermal-conductivity rubber composition and ahigh-thermal-conductivity plastic composition. Particularly, the contentis preferably 80 mass % or more.

In the present invention, no particular limitation is imposed on thetype of plastic (resin) which constitutes the aforementionedhigh-thermal-conductivity plastic composition, and any known resins canbe employed. Examples thereof include unsaturated polyester resins,acrylic resins, vinyl ester resins, epoxy resins, xylene-formaldehyderesins, guanamine resins, diaryl phthalate resins, phenolic resins,furan resins, polyimide resins, melamine resins and urea resins. Ofthese examples, unsaturated polyester resins, acrylic resins, vinylester resins and epoxy resins are preferable.

In the present invention, no particular limitation is imposed on thetype of rubber material (e.g., rubber component) that constitutes theaforementioned high-thermal-conductivity rubber composition, and anyknown rubber materials can be employed.

The present invention will be described in more detail by way ofexamples, which should not be construed as limiting the inventionthereto.

EXAMPLE 1

Added to and mixed with 500 g of a commercially available pulverizedproduct of sintered alumina (product of Alcoa Kasei Ltd.) having a meanparticle size of 51 μm were 250 g of aluminum hydroxide (product ofShowa Denko K. K.) having a mean particle size of 1 μm, 35 g of reagentgrade aluminum fluoride anhydrate and 35 g of reagent grade boric acid,and the resultant mixture was placed in a heat-resistant container madeof alumina ceramic. The mixture was heated at 1,500° C. for four hoursin a Kanthal electric furnace, and the heated product was crushed bymeans of a vibrating ball mill [100 g of fired product and 1,000 g of HDalumina balls (10 mmφ) placed in a pulverizer (model SM0.6, product ofKawasaki Heavy Industries, Ltd.)] for 30 minutes. The particle sizedistribution of the thus-crushed product was obtained through a laserdiffraction method (microtrack)

EXAMPLE 2

The procedure of Example 1 was repeated, except that the amount of addedaluminum fluoride anhydrate and that of boric acid were altered to 10 g,respectively.

EXAMPLE 3

The procedure of Example 1 was repeated, except that a pulverizedproduct of sintered alumina having a mean particle size of 90 μm wasused.

COMPARATIVE EXAMPLE 1

A commercially available pulverized product of sintered alumina (productof Alcoa Kasei Ltd.) having a mean particle size of 51 μm was placed inan amount of 500 g in a heat-resistant container made of alumina ceramicand heated at 1,450° C. for four hours in a Kanthal electric furnace.The heated product was crushed by means of a vibrating ball mill (100 gof a fired product and 1,000 g of HD alumina balls (10 mmφ) placed in apulverizer (model SM0.6, product of Kawasaki Heavy Industries, Ltd.))for 30 minutes. The particle size distribution of the thus-crushedproduct was obtained through a laser diffraction method (microtrack).

Table 1 below shows the particle size distribution and particlemorphology of the powder produced in each of Examples 1 to 3 andComparative Example 1.

TABLE 1 Example 1 Example 2 Example 3 Comp. Ex. 1 Mean 58 μm 60 μm 93 μm52 μm particle size Particle Roundish Roundish Roundish Amorphousmorphology particles particles particles particles with sharp fracture

In order to evaluate the wear property of the particles during kneadingwith resin or a similar material, materials of the formulation shown inTable 2 below were kneaded for three minutes by means of two rolls whosetemperature was predetermined at 105° C. to 115° C. Staining of theabove-formulated material caused by wear-out of hard chromium platingprovided on each roll surface was visually observed, and the degree ofstaining was rated by reference to indices.

TABLE 2 Parts Materials by mass Epoxy resin Sumiepoxy ESCN-220F (productof 100 Sumitomo Chemical Co., Ltd., softening point: 77.6° C.) Phenolicresin Sumilite Resin PR-51688 50 (product of Sumitomo Durez. Co., Ltd.,softening point: 98° C.) Hardening Imidazole C-17Z (product of 1accelerator Shikoku Chemicals Corp.) Carnauba wax Product of ToagoseiCo., Ltd. 3 Alumina 850

Specifically, the degree of migration of removed flakes (black) of hardchromium plating generated due to wear between alumina and roll surfacesinto each formulated product (white) was visually evaluated. The degreeof wear was categorized into 1) no stain, 2) faint stain, 3) slightstain, 4) considerable stain and 5) complete stain. The results areshown in Table 3 below.

TABLE 3 Example 1 Example 2 Comp. Ex. 1 Wear degree 2) 3) 5) evaluation

EXAMPLE 4

There were mixed for one hour 80 mass % of the roundish aluminaparticles produced in Example 1 and 20 mass % of commercially availablelow-soda alumina produced through the Bayer method (product of ShowaDenko K. K.) having a mean particle size of 1.7 μm by means of a rockingblender.

COMPARATIVE EXAMPLE 2

Added to 500 g of a commercially available pulverized product ofsintered alumina (product of Alcoa Kasei Ltd.) having a mean particlesize of 13 μm and a maximum particle size of 48 μm were 250 g ofaluminum hydroxide (product of Showa Denko K. K.) having a mean particlesize of 1 μm, 25 g of reagent grade aluminum fluoride anhydrate and 25 gof reagent grade boric acid, and the resultant mixture was placed in aheat-resistant container made of alumina ceramic. The mixture was heatedat 1,450° C. for four hours in a Kanthal electric furnace, and theheated product was crushed by means of a vibrating ball mill (100 g of afired product and 1,000 g of HD alumina balls (10 mmφ) placed in apulverizer (model SM0.6, product of Kawasaki Heavy Industries, Ltd.))for 30 minutes. The particle size distribution of the thus-crushedproduct was obtained through a laser diffraction method (microtrack).The produced alumina was found to assume roundish particles and have amean particle size of 18 μm.

COMPARATIVE EXAMPLE 3

There were mixed for one hour 80 mass % of the roundish aluminaparticles produced in Comparative Example 2 and 20 mass % ofcommercially available low-soda alumina produced through the Bayermethod (product of Showa Denko K. K.) having a mean particle size of 1.7μm by means of a rocking blender.

(Evaluation of Incorporation Property)

Into 100 parts by mass of silicone oil (KF 96, 1,000 cP (centipoises),product of Shin-Etsu Chemical Co., Ltd.), one of the alumina productsobtained in Examples 1 and 4 and Comparative Examples 2 and 3 wasincorporated in an amount of 500 parts by mass for one measurement andin an amount of 700 parts by mass for the other measurement. Theviscosity at 25° C. of each of the thus-prepared compositions wasmeasured by use of a B-type viscometer. The results are shown in Table 4below.

The roundish alumina particles obtained in Example 1 were identified tobe coarse corundum particles that impart favorable fluidizationcharacteristics as shown in Example 4 using the combination with themicroparticle component.

TABLE 4 Comp. Ex. Comp. Ex. Example 1 Example 4 2 3 Viscosity (P) 500parts by 600 300 1,500 400 mass 700 parts by Kneading 600 Kneading 1,800mass impossible impossible

EXAMPLE 5

Ingots prepared through electrofusion of commercial low-α-ray alumina(product of Showa Denko K. K.) having an α-ray level of 0.01 c/cm²·hr orlower were crushed, pulverized and classified under conditions whichallowed no contamination with radioactive elements to thereby yieldelectrofused alumina coarse particles having a mean particle size of 60μm and an (α-ray level of 0.005 c/cm²·hr. The alumina particles wasadded in an amount of 500 g to 250 g of low-α-ray aluminum hydroxideobtained through a known method (product of Showa Denko K. K.) having anα-ray level of 0.005 c/cm²·hr and a mean particle size of 5 μm, and theresultant mixture was fired and crushed in a manner similar to that ofExample 1 to thereby yield roundish alumina particles having an α-raylevel of 0.004 c/cm²·hr and a mean particle size of 67 μm.

INDUSTRIAL APPLICABILITY

As described hereinabove, the corundum particles produced through theprocess of the present invention are roundish, coarse particles thatprovide substantially no problematic wear of machines and apparatus.When the particles are mixed with a microparticle component to therebywiden the particle size distribution, there can be produced a resincomposition containing a large amount of filler, which has never beenproduced due to poor fluidity.

1. A process for producing roundish alumina particles, comprising:heating at 1,000 to 1,6000 °C. a composition containing at least one ofelectrofused alumina and sintered alumina having a mean particle sizegreater than 35 μm, an alumina hydrate and at least one species selectedfrom the group consisting of a halogen compound and a boron compound;and crushing the composition; wherein the alumina hydrate is added in anamount of 50 to 300 mass % based on said at least one of electrofusedalumina and sintered alumina as reduced to alumina.
 2. The processaccording to claim 1, wherein the electrofused alumina and sinteredalumina have a mean particle size of at least 50 μm.
 3. The processaccording to claim 1, wherein the electrofused alumina and sinteredalumina have a mean particle size greater than 35 μm and less than orequal to 120 μm.
 4. The process according to any one of claims 1 to 3,wherein at least one of the halogen compound and the boron compound isadded in an amount of 5 to 20 mass% based on an entire amount ofalumina.
 5. The process according to claim 1, wherein the halogencompound is at least one species selected from the group consisting ofAlF₃, NaF, CaF₂, MgF₂ and Na₃AlF₆.
 6. The process according to claim 1wherein the boron compound is at least one species selected from thegroup consisting of B₂O₃, H₃BO₃, mNa₂O.nB₂O₃, wherein each of m and n isan integer, and a borofluoride compound.
 7. The process according toclaim 1, wherein the alumina hydrate is at least one species selectedfrom the group consisting of aluminum hydroxide, alumina gel, amorphousaluminum hydroxide and a partial hydrate of aluminum compound.
 8. Theprocess according to claim 1 , wherein each of the electrofused alumina,sintered alumina and alumina hydrate has an x-ray level of 0.01 c/cm²·hror less.
 9. Roundish alumina particles produced through the processaccording to claim
 1. 10. A high-thermal-conductivity rubber compositioncontaining the roundish alumina particles according to claim
 9. 11. Ahigh-thermal-conductivity plastic composition containing the roundishalumina particles according to claim
 9. 12. A sealing material forelectronic parts comprising the roundish alumina particles according toclaim
 9. 13. The sealing material according to claim 12, wherein theelectronic parts are high-integration Ics, LSIs or VLSIs.
 14. A finishlapping material comprising the roundish alumina particles according toclaim
 9. 15. Aggregates incorporated into refractory, comprising theroundish alumina particles according to claim
 9. 16. Glass comprisingthe roundish alumina particles according to claim
 9. 17. Ceramiccomprising the roundish alumina particles according to claim 9.